The present invention relates to a circuit arrangement for switching on a partial circuit arrangement, the circuit arrangement having a first switching element with a control electrode and a reference electrode, the reference electrode being connected to a reference potential. It further comprises a partial circuit arrangement, the first switching element having to be activated when the circuit arrangement is being switched on, and at least one second switching element, which is required for the operation of the circuit arrangement after the partial circuit arrangement has been switched on but which is not assigned any activity when the partial circuit arrangement is being switched on, the at least one second switching element having a control electrode and a reference electrode, which is connected to the reference potential. It further comprises an activation circuit for the first switching element, the activation circuit comprising a storage capacitor which, to activate the first switching element, is connected via a DIAC to the control electrode of the first switching element, and a first diode which is arranged between the reference potential and the storage capacitor in such a way that it is made possible for a current to flow to activate the first switching element.
In order to clarify the problem on which the invention is based, such a circuit arrangement, disclosed by the prior art, is illustrated in FIG. 1. It is used by the applicant of the present invention as a starting circuit for lamp operating devices which have a half-bridge arrangement. In particular as a starting circuit for a freely oscillating converter, one of the two half-bridge transistors initially has to be switched on. In the case of a freely oscillating converter, which is to be assumed here by way of example in order to illustrate the invention, the two half-bridge transistors are driven via their control electrode, in actual operation, only after a separate starting operation. In FIG. 1, the transistor T1 represents one of the two half-bridge transistors. The transistor T2 is a second switching element, which is required for the operation of the circuit arrangement after it has been switched on, but which is assigned no activity while the partial circuit arrangement is being switched on, here the lower half of the freely oscillating converter for operating the lamp. The bases of the transistors T1 and T2 are connected to each other via a resistor R1. In order to switch on the transistor T1, that is to say to start an oscillation, a pulse-like switching-on signal is necessary. In the present case, this is achieved by a capacitor C1, which on one side is connected via at least one resistor R to the positive signal +(NGR) from a mains rectifier and on the other side is connected via a resistor R2 to ground, being charged up. This capacitor voltage is present on one side of a DIAC, whose other terminal is connected to the control electrode of transistor T1. Then, as soon as the voltage on that terminal of the DIAC which is connected to the capacitor C1 exceeds a certain limiting value, said DIAC breaks down and permits a sudden current surge to the control electrode of the transistor T1. By this means, the transistor T1 is switched on, and therefore the freely oscillating converter is started. The firing current for starting the freely oscillating converter initially flows in the circuit comprising DIAC, transistor T1, reference potential, resistor R2 and capacitor C1. However, as the firing current grows, the voltage drop across the resistor R2 at some time becomes so high that the firing current experiences a lower resistance if it flows via the diode D1. The current flow then changes from the resistor R2 to the diode D1.
The task of the transistor T2 begins after the firing of the transistor T1. In the present example, it consists in blocking the DIAC during normal operation, since repeated firing of the DIAC would disrupt the continuous operation of the freely oscillating converter. The latter functions in such a way that the base signal of transistor T1 is also applied via R1 to the base terminal of the transistor T2. The collector of transistor T2 is connected to the potential between storage capacitor C1 and DIAC. During normal operation, the transistor T1 is driven via the line BT1. This signal is also applied to the base of the transistor T2 via R1.
The storage capacitor C1 is therefore discharged regularly via T2, and no disruption to the operation of transistor T1 occurs.
The disadvantage of this known circuit arrangement is that precisely at the time at which the full energy stored in the storage capacitor C1 leads to the breakdown of the DIAC, some of the energy is led past the envisaged locationxe2x80x94namely the control electrode of the transistor T1xe2x80x94via R1 to the base of transistor T2. This leads to the transistor T2 turning on and, via its operating electrode, dissipating some of the energy stored in the storage capacitor C1 to the reference potential. As a consequence, it can be established that not all of the energy stored in C1 is available for firing the transistor T1, but is dissipated via a switching element which is not assigned any activity during the actual firing operation. As explained above, the purpose of transistor T2 is based on normal operation, that is to say after the firing of transistor T1.
The negative result of this is that the storage capacitor C1 has to be dimensioned to be considerably larger, which in turn results in the entire circuit arrangement being slowed down.
The object of the present invention is therefore to develop a generic circuit arrangement in such a way that the disadvantages of the prior art are overcome, in particular the provision of a faster circuit arrangement is made possible.
According to the invention, this object is achieved in that, in series with the first diode, on the side of the first diode facing away from the reference potential, a second diode is arranged in the same orientation as the first diode, the junction between the first and the second diode being connected to the control electrode of the at least one second switching element and, in series in a connection between the control electrode of the second switching element and the reference potential, in addition to the second diode, at least one third diode is connected in the same orientation as the second diode.
The invention is based on the fundamental idea of switching off at least one second switching element actively via the DIAC at the same time as the first switching element is initially switched on, the switching-off action also being effected by the DIAC. In particular, in order to switch off the at least one second switching element, use is made of the current which flows through the storage capacitor C1 belonging to the DIAC when the latter is fired. Therefore, the at least one second switching element is switched off by exactly the same current by which the first switching element T1 is switched on. This implementation is optimal with regard to the cost question, since no further controllers, timing elements etc. are needed. It is in particular independent of component parameters and can therefore preferably also be used in mass production. Moreover, it is naturally real-time capable. With minimum expenditure, the invention provides an extremely robust and exactly functioning solution.
A preferred embodiment of the invention is distinguished by the fact that the side of the series circuit comprising the second and third diode and facing away from the control electrode of the second switching element-is connected firstly to the storage capacitor and secondly via a resistor to the reference potential. This measure means that the firing operation is boosted since, when the voltage drop across the resistor becomes greater than the sum of the diode forward voltages, the diodes take over the firing current.
A further preferred embodiment is distinguished by the fact that the junction between the second and third diode is connected to the storage capacitor, and the third diode is connected by its other terminal to the reference potential. In the case of this variant, the charging current for the storage capacitor flows through the third diode, and the discharge current flows via a path which comprises D1 and D2. As compared with the exemplary embodiment previously described, this results in the advantage that, lacking a resistor, the residual current through the resistor is also dispensed with. The energy present in the storage capacitor C1 is therefore used more effectively for the firing operation.
A circuit arrangement according to the invention can also comprise a plurality of second switching elements, in particular a number xe2x89xa72, the reference electrodes of all the second switching elements being connected to the same reference potential, each second switching element being assigned a second diode, and the sides of the respective second diodes facing away from the control electrode of the respective second switching element being interconnected, so that the third diode acts for all the second switching elements. The advantage of this embodiment is that a single third diode acts for a large number, in particular for all, of the second switching elements.
A further cost reduction is permitted by an embodiment which has m first diodes, where 1xe2x89xa6m xe2x89xa6n, the association in each case between a first diode and the n second switching elements being as desired. By means of this measure, the reduction to a single first diode is made possible in the case of a plurality of second switching elements.
At least one second switching element can have an operating electrode which is coupled to the line between the storage capacitor and the control electrode of the first switching element. The measures according to the invention are desirable in particular in the case of such wiring of a second switching element, since in the case of a connection of this type of a second switching element, there is a particularly high risk that the energy envisaged for the activation of the first switching element will flow away unused via the second switching element. The same is true of the case in which at least one second switching element is coupled by its control electrode to the line between the storage capacitor and the control electrode of the first switching element.
In order to charge the storage capacitor, it can be arranged in series with at least one resistor between a voltage source and the reference potential.
According to a further aspect of the present invention, an operating device for a lamp is also provided which has a circuit arrangement according to the invention, the circuit arrangement comprising a half-bridge arrangement with two half-bridge transistors, and the first switching element being one of the two half-bridge transistors.
Further advantageous embodiments emerge from the subclaims.